In a copying machine using an electrophotographic system, and an image formation apparatus such as a laser printer, a corona discharger is in general use for charging an image bearing member, that is, a photoconductor, for performing image transfer, for separating copy paper, or for quenching charges of the photoconductor, since uniform charging can be performed by the corona charger.
However, since a high voltage as high as 4000 to 8000 V is applied to the corona discharger, a large amount of ozone (O.sub.3) is generated by the corona discharger when discharging is carried out. Effects of ozone on living body system are described in detail in an article entitled "Troubles caused by corona discharging and techniques for preventing the troubles" (Electrophotography 30.3/1991) by Yasuyuki Tabata. Nitrogen oxides (Nox) which are generated at the same time as the generation of ozone are also considered to work as factors by which the quality of produced images is degraded and the life of the photoconductor is shortened, since such nitrogen oxides are deposited on the surface of the photoconductor or penetrate into the photoconductor.
In particular, nitrogen oxides (Nox=NO, NO.sub.2, NO.sub.3, . . . ) react with water in air to produce nitric acid (HNO.sub.3) which is deposited not only on the photoconductor, but also on and around the charging apparatus itself, and corrodes metallic materials used in the charging apparatus. As a result, the characteristics of the photoconductor are caused to deteriorate (refer to Japanese Laid-Open Patent Applications 61-12358, 62-7065 and 2-79069). Therefore, as countermeasures for controlling the generation of corona products from the corona discharger, many proposals have been reported, such as locally providing an ozone absorbing material on a shield wall for the discharger, providing an ozone decomposing material in vicinity with the photoconductor, and heating the discharger. However, currently such proposals cannot always sufficiently cope with the above-mentioned problems, since those proposals have problems with respect to the continuity of the effect, the cost, or the capability of completely nullifying ozone products. Under such circumstances, in recent years, there have been made studies as to how to make practically usable contact charging methods (such as roller charging method and brush charging method) which are considered to be methods with an extremely small amount of generation of ozone, and such a method has already actually been put to practical use in some copying machines.
FIG. 17 shows an outline of a conventional corona charger. In the figure, reference number 1 indicates a photoconductor; 50, a shield case; 51, a charge wire; and 52, a grid. Contact charging apparatus are shown in FIG. 18 to FIG. 20. FIG. 18 shows a brush contact charging apparatus. In the figure, reference numeral 53 indicates an electroconductive brush. FIG. 19 shows a blade contact charging apparatus. In the figure, reference numeral 54 indicates a core material, and 55 indicates an electroconductive unwoven cloth. Furthermore, FIG. 20 shows a roller contact charging apparatus. In the figure, reference numeral 56 indicates a resistor layer, and 57 indicates a surface protective layer. In the charging apparatus shown in FIG. 18 and FIG. 19, there are generally used electroconductive fibers which are carbonized to such a degree that the resistance thereof is, for instance, in the range of about 10.sup.3 to 10.sup.6 .OMEGA.; and in the charging apparatus shown in FIG. 20, there is generally used an electroconductive rubber roller with the resistance thereof being adjusted to about 10.sup.5 .OMEGA..multidot.cm by uniformly dispersing carbon powder in the rubber.
The corona discharger generates ozone in an amount of about 1 to 10 ppm, and nitrogen oxides (Nox) such as NO.sub.2 and NO.sub.3 in an amount of about 0.05 to 0.5 ppm immediately below a charger thereof in a copying machine with a copy speed of about 10 to 40 cpm, provided that the amounts of such products generated change depending upon the copy speed. In the contact charging method (for instance, using the roller charging apparatus), 1500 to 2000 V, which is about 1/4 the voltage applied in the corona discharging method, is applied to the roller charging apparatus, so that the amount of ozone generated in the contact charging method is extremely small in the range of 0.02 to 0.5 ppm, and the amounts of nitrogen oxides (Nox) are also as small as about 0.03 to 0.05 ppm.
Thus, the contact charging method is directed to the improvement with respect to the above-mentioned problem.
However, the amounts of the above-mentioned corona products are not zero, so that the corona products are gradually accumulated and deposited on an image bearing member. In the case of an organic photoconductor without a protective layer, the surface is scraped off, so that the deposited products can be relatively easily removed and therefore the effects of such products on the organic photoconductor are small. However, when the organic photoconductor is used for an extended period of time, the contamination of the photoconductor with the corona products gradually spreads into a photoconductive layer thereof with time, and eventually the deterioration of image quality is brought about. When the photoconductor is coated with a protective layer with high hardness from which contamination products are difficult to remove, such as a DLC film (diamond-like carbon film), a period of time free of problems can be significantly extended in comparison with such time as in the corona discharging method. However, when the photoconductor is exposed to corona products for a long period of time, image quality deterioration such as image flow tends to take place easily. Such contamination takes place more or less even when the hardness of the OPC layer is increased.
Each of Japanese Laid-Open Patent Applications 6-274007 to 274009 discloses the use of an electroconductive unwoven cloth (10.sup.5 to 10.sup.7 .OMEGA./cm.sup.2) as a contact member which comes into contact with the photoconductor. When fibers such as acrylic fibers and pitch fibers are burned at high temperatures, such fibers are carbonized and become electroconductive and therefore can be used as materials for a contact charging member. The resistance value thereof varies depending upon the burning temperature. However, when the contact charging member is in the shape of an unwoven cloth, even if the contact charging member comes into contact with the image bearing member, there is no risk that deep scratches are formed in the image bearing member, since the contact member is an unwoven cloth made of extremely thin fibers, just like a felt. However, the ends of the fibers are bare and such fibers that can be used in the charging apparatus are thin, with a small fiber diameter, because of the necessity for the uniformity in the thickness of the fibers. Therefore such fibers tend to be easily pulverized, and pulverized fibers are deposited around the charging apparatus, causing abnormal discharging, and when such powder enters a cleaning section or a development section, the photoconductor and a cleaning blade are scratched, so that troubles such as lowering image quality, for instance, due to resolution reduction or improper image transfer, tend to be caused.
Japanese Laid-Open Patent Application 7-5745 discloses a contact charging apparatus which is in the shape of a band or a roller made of an electroconductive resin film which is prepared by dispersing electroconductive fibers with a water absorption of 0.2% or less in a resin.
Japanese Patent Publication 6-75221 discloses the use of a locally carbonized polyacrylonitrile fiber (with a tip portion thereof being treated so as to have a resistance of 10.sup.2 to 10.sup.6 .OMEGA..multidot.cm) in a contact portion with a photoconductor.
As mentioned above, the electric resistance of the fibers depends upon the burning temperature of the fibers, that is, there is the tendency that the higher the burning temperature, the lower the electric resistance. Therefore, the resistance can be obtained as desired by appropriately adjusting the burning temperature. However, the fibers tend to become brittle as the burning temperature is increased.
A charging member used in the above-mentioned conventional art is composed of carbon fibers with an electroconductivity set in a certain range, and therefore ozone and nitrogen oxides which are generated at the time of charging cannot be absorbed on the carbon fibers themselves.
Therefore it is an object of the present invention to provide a charging member having a function of removing contamination materials such as ozone and nitrogen oxides with high efficiency and capable of preventing the deterioration of the characteristics of the image bearing member and maintaining stable image quality over an extended period of time, and to provide a charging apparatus using the above-mentioned charging member.